Polycarbonate resins form many blends with emulsion derived vinyl polymers that have useful features, such as high impact, high melt flow, good appearance and improved solvent resistance. However the nature of the vinyl polymers made by an emulsion process creates specific issues with the stability of the polycarbonate (PC) resin used in blends with them. PC is a condensation polymer with carbonic acid ester repeat units. These carbonate linkages can react with water causing the polymer to lose molecular weight and ultimately physical properties. Various catalysts, such as acids and bases or chemical remnants of the emulsion polymerization process that may generate undesired catalysts when the blend is melt processed by molding or extrusion, may increase the rate of PC hydrolysis. PC resins are relatively sensitive to degradation and are somewhat unusual for condensation polymers in that when they decompose they can give off carbon dioxide, derived from the carbonate linkages. Carbon dioxide generation can cause the PC blend to foam or give a plastic part whose surface is marred by splay, due to carbon dioxide. Such decomposition leaves behind a phenolic end group.
The use of specific process aids such as emulsifiers and coagulation agents in emulsion polymerization process may cause decomposition of PC. For instance, residues of the emulsifier, such as fatty carboxylic acids or their salts can cause issues, with PC stability. In addition emulsion polymerized resins must be separated from the water in which they are made. This separation is frequently done by coagulation; addition of salt water or acid is often used, along with filtration, to separate the emulsion polymer from water. Despite this separation the emulsion polymers often contain varying minor amounts of residues that may cause PC instability. In many cases it is not industrially or economically feasible to totally purify the emulsion polymer from such by-products of its manufacture. For instance in an acid or salt coagulated emulsion polymer, excessive washing may be needed to purify the polymer such that it can be used in a PC blend in demanding conditions. However this extra washing may require excessive water leading to increased pollution and or higher treatment costs. In other instances the use of emulsion polymerization adjuncts, such as emulsifier and radical initiators is critical to a successful emulsion polymerization and cannot be removed or substituted for. Therefore it would be beneficial to be able to use the emulsion polymers, as they are isolated from polymerization, in combination with PC. However in demanding applications, such as exposure to humidity at high temperature, the residues of the emulsion polymerization process can lead to PC breakdown.
Accordingly, it would be beneficial to provide polycarbonate compositions that have improved hydrolytic stability.